interactive bots: Create converter bot.

contrib_bots/lib/ConverterBot/docs.md

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+# Converter bot
+
+This bot allows users to perform conversions for various measurement units.
+
+## Usage
+
+Run this bot as described in [here](http://zulip.readthedocs.io/en/latest/bots-guide.html#how-to-deploy-a-bot).
+
+Use this bot with the following command
+
+`@convert <number> <unit_from> <unit_to>`
+
+This will convert `number`, given in the unit `unit_from`, to the unit `unit_to`
+and print the result.
+
+* `number` can be any floating-point number, e.g. 12, 13.05, 0.002.
+* `unit_from` and `unit_to` are two units from [the following](#supported-units) table in the same category.
+* `unit_from` and `unit_to` can be preceded by [these](#supported-prefixes) prefixes.
+
+### Supported units
+
+| Category | Units |
+| ----------------- | ----- |
+| Area | square-centimeter (cm^2, cm2), square-decimeter (dm^2, dm2), square-meter (m^2, m2), square-kilometer (km^2, km2), square-inch (in^2, in2), square-foot (ft^2, ft2), square-yard (y^2, y2), square-mile (mi^2, mi2), are (a), hectare (ha), acre (ac) |
+| Information | bit, byte |
+| Length | centimeter (cm), decimeter (dm), meter (m), kilometer (km), inch (in), foot (ft), yard (y), mile (mi), nautical-mile (nmi) |
+| Temperature | Kelvin (K), Celsius (C), Fahrenheit (F) |
+| Volume | cubic-centimeter (cm^3, cm3), cubic-decimeter (dm^3, dm3), liter (l), cubic-meter (m^3, m3), cubic-inch (in^3, in3), fluid-ounce (fl-oz), cubic-foot (ft^3, ft3), cubic-yard (y^3, y3) |
+| Weight | gram (g), kilogram (kg), ton (t), ounce (oz), pound (lb) |
+| Cooking (metric only, U.S. and imperial units differ slightly) | teaspoon (tsp), tablespoon (tbsp), cup |
+
+### Supported prefixes
+
+| Prefix | Power of 10 |
+| ------ | ----------- |
+| atto | 10<sup>-18</sup> |
+| pico | 10<sup>-15</sup> |
+| femto | 10<sup>-12</sup> |
+| nano | 10<sup>-9</sup> |
+| micro | 10<sup>-6</sup> |
+| milli | 10<sup>-3</sup> |
+| centi | 10<sup>-2</sup> |
+| deci | 10<sup>-1</sup> |
+| deca | 10<sup>1</sup> |
+| hecto | 10<sup>2</sup> |
+| kilo | 10<sup>3</sup> |
+| mega | 10<sup>6</sup> |
+| giga | 10<sup>9</sup> |
+| tera | 10<sup>12</sup> |
+| peta | 10<sup>15</sup> |
+| exa | 10<sup>18</sup> |
+
+### Usage examples
+
+| Message | Response |
+| ------- | ------ |
+| `@convert 12 celsius fahrenheit` | 12.0 celsius = 53.600054 fahrenheit |
+| `@convert 0.002 kilomile millimeter` | 0.002 kilomile = 3218688.0 millimeter |
+| `@convert 31.5 square-mile ha  | 31.5 square-mile = 8158.4625 ha |
+| `@convert 56 g lb` | 56.0 g = 0.12345887 lb |
+
+## Notes
+
+* You can use multiple `@convert` statements in a message, the response will look accordingly:
+![multiple-converts](multiple-converts.png)
+
+* Enter `@convert help` to display a quick overview of the converter's functionality.
+
+* For bits and bytes, the prefixes change the figure differently: 1 kilobyte is 1024 bytes,
+1 megabyte is 1048576 bytes, etc.